Environmental Engineering Reference
In-Depth Information
Figure 5.5.
Analysis of the sensitivity of loggerhead sea turtle populations to 50 per-
cent improvements in survival of different stage classes. The graph shows the effect of
changing the survival of one particular age class (bars) on total population growth rate
relative to a baseline (dashed line) that uses average parameter values (Box 5.1). For
example, improving egg survival will increase the growth rate of the population from a
baseline of -0.42 to -0.35. But, such improvement should not stem the population
decline. Alternatively, improving large juvenile survival should lead to a positive popula-
tion growth rate of 0.12. The analysis suggests that loggerhead population declines are
best reversed by focusing conservation efforts at improving survival of large juveniles and
subadults, not the egg stage. The graph is based on information provided by Crouse et
al. (1987).
policy related to the use of TEDs (Crowder et al. 1994).There are two basic
management options for the fishery. Fisherman might use the TEDs only
during the shrimping season and only
offshore. Or they could be forced to
use TEDs year round and in all wa-
ters. An analysis of the options, using
structured modeling revealed that
loggerhead sea turtle populations
would take seventy years or more to
increase an order of magnitude in size
if trawl fisheries used TEDs only dur-
ing the shrimping season and only
The modeling does not provide evi-
dence of cause-effect: The prescrip-
tions are merely hypotheses formu-
lated as management options.
Thus, careful monitoring should fol-
low any implementation of a man-
agement option to ensure that the
modeling predictions are borne out.
